BESPIRATION, AND ANIMALlZATiO.N, 



phcity in molluscous animals. The heart of the teredo has two auricles 

 and two ventricles ; that of the oyster one auricle and one ventricle. In 

 the muscle the heart is not, strictly speaking, divided into an auricle and 

 ventricle, but rather consists of an oval bag, through the middle of which 

 the lower portion of the intestine passes. Two veins from the gills open 

 into the heart, one on each side, which may be considered as the auricles. 



In several of the crustaceous insects of Linneus, as, for example, the 

 monoculus and craw-fish, the stigmata converge into a cluster, so as to 

 form gills ; which in some species are found seated in the claws, and in 

 other species under the tail. These have for the most part a small single 

 heart, and consequently a single circulation, the course of which, however, 

 is directly the reverse of that pursued in fishes ; for the heart in the present 

 instance propels the blood through the body, and the gills receive it, and 

 propel it to the heart. This is also the case in the snail, slug, and many 

 other soft-bodied worms, which possess a gill in the neck consisting of a 

 single aperture, which it can open or shut at pleasure. Yet, with a sin- 

 gular kind of apparent sportiveness, the cuttle-fish is possest of three dis- 

 tinct hearts, which is one more than is allotted to mankind, in whom this 

 organ is only double. 



In zoophytes we are in great ignorance both as to their sanguineous 

 and respiratory functions. That they stand in need of oxygene, and even 

 of nitrogene, has been sufficiently determined by Sir H. Davy ; as it has 

 also that they absorb their oxygene and nitrogene, as fishes do, from the 

 water which holds these gases in solution. Their nutrition appears to be 

 effected by an immediate derivation of the nutritive fiuid from their interior 

 cavity into the gelatinous substance of their body.* 



Hence, then, the respiratory organs of the animal kingdom may be di- 

 vided into three classes ; lungs, gills, and holes or stigmata : each of the 

 three classes exhibits a great variety in its form, but the office in which 

 they are employed is the same. Animals of every kind must be supphed 

 with air, or rather with oxygene, however they may difl^er in other respects 

 in tenacity of life ; for a vacuum, or a medium deprived of oxygene, 

 kills them equally. Snails and slugs corked up in small bottles have been 

 found to live till they had exhausted the air of every particle of oxygene, 

 and to die immediately afterwards : and frogs and land-turtles, which are 

 well known to survive the loss of the spinal marrow for months, and that 

 of the head or heart for several days, die almost instantly on exposure to 

 a vacuum.! 



Connected with this general subject, there is still an important question 

 to be resolved, and which has greatly occupied the attention of physiolo- 

 gists for the last fifty years. 



Mediately or immediately, almost all animal nutriment, and consequently 

 almost all animal organization, is derived from a vegetable source. The 

 blade of grass becomes a muscular fibre, and the root of a yam or a po- 

 tato a human brain. What, then, is that wonderful process which assi- 

 milates substances in themselves so unhke ? that converts the vegetable 

 into an animal form, and endows it with animal powers ? 



Now, to be able to reply succinctly to this question, it is necessary first 

 of all to inquire into the chief feature in which animal and vegetable sub- 

 stances agree, and the chief feature in which they diffier. 



Animals and vegetables, then, agree in their equal necessity of extract 



Blumenbach, § 167, t See Encyclop. Brit. art. Phvsiol. p. 679 



I 



